Add a limit on returned matches

In case there are several instances of the referenced entity that match the user search entry, the server may time out and throw a 500 error. I updated the code to limit the number of returned results to 100. I suggest making this configurable in settings.

In entityreference_autocomplete_autocomplete_callback:
before:

$query->propertyCondition($label_column, '%' . $last_label . '%', 'LIKE');
$result = $query->execute();

after:

$query->propertyCondition($label_column, '%' . $last_label . '%', 'LIKE');
$query->range(0, 100);
$result = $query->execute();